I was on Perseus, checking out what form [face=SPIonic]pro/es [/face] (Iliad 1:127) is and it said: "aor imperat act 2nd raw_preverb.
I don't have any idea what 'raw_preverb' means. Can someone tell me please?

The term 'preverb' means an element, typically a preposition or augment, prefixed to a verb. I think Perseus must be refering to the [face=SPIonic]pro/[/face] in [face=SPIonic]pro/ej[/face]. The latter is 2nd singular aorist2 imperative active of [face=SPIonic]proi+/hmi[/face].

Raw preverb simply means that the combining element hasn't changed phonetically. You're right on there. The case of ἀμπέχω is wholly different. This is explained by Grassmann's law (applicable in Greek and Sanskrit), which states that when two aspirated stops occur in the same word, the first is dissimilated to its unaspirated equivalent. So ἀμφ' + ἔχω produces ἀμπέχω.

Oh, is that what it means?! I came across that term (raw preverb) in my reading a while ago, but I couldn't make heads or tails of what the book was talking about... It makes so much sense when you explain it that way.